How to undo split normals?
@scurest had the correct answer for this one. With Blender version 2.92 it seems undoing Mesh - Normals - Split is a bit harder:

Select the object and click &quot;Clear Custom Split Normals Data&quot; in properties editor - Object Data Properties - Geometry Data. (See image below.)
Press TAB to enter Object Edit mode (or select it from viewport top left corner).
Press key a to select all (or use Select - All from menu).
Select Edge - Clear Sharp from the viewport top menu.

It seems that with Blender version 2.92 the action Mesh - Normals - Split does both adding custom split normals data and marking all affected edges as sharp. To undo that, both must be fixed.
This is documented at https://docs.blender.org/manual/en/2.92/modeling/meshes/structure.html#modeling-meshes-normals-custom